This invention relates to measuring magnetic fields produced by living organisms, and, more particularly, to determining the sources of such biomagnetic activity from external measurements.
Living organisms produce magnetic fields that can be measured noninvasively with sensors positioned outside of the organism. These magnetic fields arise from electrical activity within the organism. Measurement of the magnetic fields can lead to an understanding of the electrical activity. For example, the measurement of magnetic fields produced by the brain can lead to an understanding of the mechanisms of perception and sensory response, as well as to the normal functioning of the brain and conditions that lead to illnesses and abnormalities.
Biomagnetometers are devices that measure the small magnetic fields produced by a living organism. The biomagnetometer typically includes a number of sensors arranged in an array external to the organism, which measure the magnetic field at a number of locations. Each sensor typically has a magnetic field pickup coil that may be a magnetometer or a gradiometer. When a small magnetic flux change penetrates the pickup coil, a small electrical current flows in the coil. This small current is detected by a sensitive detector of electrical currents, preferably a Superconducting Quantum Interference Device, known by the acronym "SQUID". The output of the various SQUIDs, after amplifying, filtering, and signal conditioning, is provided to a computer which stores and analyzes the data. The SQUIDs operate only at superconducting temperatures, and to attain the best system performance the pickup coils and SQUIDs are usually placed into a cryogenically cooled dewar. Because the biomagnetic fields produced by the body are so small compared to the magnetic fields of the earth and many types of electrical apparatus, it is common to place the subject of the biomagnetic study into a magnetically shielded room that excludes external magnetic and radio frequency fields.
The course of development of biomagnetometers has led to ever-increasing numbers of sensors (pickup coils and SQUIDs) in each unit. As of this writing, biomagnetometers with 37 sensors are available commercially, and even larger numbers of sensors are likely in future units. One impetus to increasing the number of sensors is that the larger amount of information available from the array of sensors offers the promise of magnetically imaging the source region during operation of the magnetic fields. That is, with a sufficient amount of information it becomes possible to form pictures or maps of activity in organs such as the human brain and heart.
The fundamental problem in analyzing sources of magnetic activity in the brain within the context of operation of a biomagnetometer is that the number of potential sources is greater than the number of sensors. Exact solutions are therefore not possible. Additionally, the magnetic field produced within the organism passes through several media (e.g., tissue, bone, air) before being measured, thereby complicating the analysis. In order to produce optimal solutions within these constraints, various analytical techniques to solving this "inverse problem of biomagnetism" have been utilized.
One common analytical approach has been to assume that the externally measured magnetic field is produced by a single dipole source or some small number of dipole sources, and to calculate the location, orientation, and strength of the source(s) from the external measurements. This approach is questionable because of its oversimplification of the physiology of the organism, but has been useful in a number of contexts. More rigorous approaches involve mathematical analyses of the data gathered by the sensors, such as the lead field synthesis technique described in U.S. Pat. No. 4,977,896. The minimum norm estimation technique also shows promise, because it provides a solution optimized according to physical principles. However, it may not yield fully acceptable results in some circumstances.
There is therefore a need for an improved biomagnetometer and approach to making biomagnetic measurements wherein the magnetic field sources within the subject organism are imaged based upon the external measurements of magnetic field. The present invention fulfills this need, and further provides related advantages.